Known techniques for the production of metallic soaps are roughly divided into a fusion process and a double decomposition process. The fusion process comprises directly reacting a fused fatty acid and a metal oxide or a metal hydroxide to obtain a metallic soap. The double decomposition process comprises reacting an aqueous solution of an alkali soap comprising an alkali metal salt or ammonium salt of a fatty acid (hereinafter simply referred to as "alkali soap") with an inorganic metal salt.
Although the fusion process has merits relating to reactor vessels in that the steps involved are simple so that small-sized reactor vessels suffice for carrying out the process, it has a number of disadvantages: (a) The reaction hardly reaches completion so that the resulting metallic soap contains unreacted fatty acid and metal oxide or hydroxide in considerable quantities. (b) Since the reaction proceeds in high temperatures, the resulting metallic soap is slightly colored. (c) The metallic soap obtained is contaminated with different metals derived from the starting metal oxide or metal hydroxide, e.g., iron, lead, cadmium, and manganese, to a not inconsiderable extent. (d) The product obtained generally has a large particle size, and high labor is required for grinding the coarse particles to obtain a desired particle size.
To the contrary, the double decomposition process has merits relating to product quality, i.e., reduced contents of unreacted materials and different metals, a satisfactory hue, and fineness of the product. It involves, however, disadvantages: (a) Large-sized reactor vessels are required. (b) The reaction slurry shows unstable dispersibility in water, thus deteriorating workability. (c) Water-soluble salts remain in the resulting metallic soap.
Hence, various processes for overcoming the above-described disadvantages associated with the fusion process or double decomposition process have been studied. For example, it has been recommended to carry out the fusion process in the presence of a surface active agent as disclosed in JP-B-60-21573 (the term "JP-B" as used herein means an "examined published Japanese patent application") or under reduced pressure as disclosed in JP-B-61-39296. Even by these proposals, however, the unreacted fatty acid and metal oxide or metal hydroxide as raw materials still remain in the product, though not so serious as in the conventional direct process, failing to obtain product quality on the same level as attained in the double decomposition process. Similarly, it has been suggested to carry out the double decomposition process in the presence of a surface active agent as disclosed in JP-B-51-44002. However, the product cannot be obtained without contamination of the surface active agent used. While the slurry concentration can be increased twice as much as that of the conventional double decomposition process, since the process is carried out in a batch-wise operation, large-sized reactor vessels are still required.
Several processes for continuously preparing metallic soaps have hitherto been reported. For example, British Patent 693,741 describes a continuous process for producing an aluminum soap by double decomposition. This process is carried out using four vessels arranged in series with overflow connections. To a first vessel are continuously fed an aqueous sodium soap solution and an aqueous solution of aluminum sulfate at [Al.sub.2 (SO.sub.4).sub.3 ] substantially equivalent feed rates (assuming that 2 mols of a carboxyl group react per mol of an aluminum atom) to conduct a double decomposition reaction. The reaction product is then forwarded to a second vessel, where an excess of an aqueous solution of Al.sub.2 (SO.sub.4).sub.3 is added [11 to 13% excess, calculated as aluminum oxide (Al.sub.2 O.sub.3)]. The reaction product is further forwarded to a third vessel and then to a fourth vessel continuously and aged thereby to increase the particle size of an aluminum soap in a easily filterable form.
According to this process, the reaction in the first vessel should be carried out with vigorous agitation with no reference to the structure of the reaction vessel to be used. That is, the production process is designed so that the reaction slurry continuously passes through batchwise reaction vessels connected in series. Therefore, the process enjoys no merit of size reduction of plant which should have been resulted from a continuous process. Further, it is described that the particle size is increased while mixing at an elevated temperature. However, from the fact that the mixing is carried out with the starting metal salt, Al.sub.2 (SO.sub.4).sub.3, being present in a large excess, this process cannot be regarded practical taking into consideration removal of the excess of the starting material in the working-up stage, an increase of the metal content in the final product, and a consumption of the starting material.
JP-A-56-169642 (the term "JP-A" as used herein means an "unexamined published Japanese patent application") describes a process for continuously preparing metallic soaps by a wet fusion process, comprising mixing an aqueous emulsion of a fatty acid and an aqueous dispersion of a basic metal carbonate under a high shearing condition to continuously obtain an aqueous slurry of a metallic soap. An aqueous slurry of a metallic soap is a so-called Bingham fluid which has a low viscosity with fluidity with only the external force applied, but increases its viscosity and loses fluidity when released from the external force.
The apparatus employed in the above-described wet fusion process has such a structure that the two liquids as raw materials may meet in a pipe and then be forwarded to a part where a high shear force is imposed. In other words, the two liquids are not fed directly to the part under a high shear force. It is considered therefore that a metallic soap is generated in the place where the liquids meet and the slurry containing the thus generated metallic soap increases in viscosity to lose fluidity, which would clog the pipe, ultimately resulting in a failure of continuously using the apparatus. Besides, since the apparatus uses a mixer having no self-discharge ability without any consideration given to discharge of the aqueous slurry after mixing under a high shearing condition, there is a fear that the mixer is accumulated with the thickened slurry and finally fails to be used. From all these considerations, when such an apparatus is applied to a double decomposition process, it is impossible to perform continuous production due to clogging.
Further, this process has a problem of incompleteness of the reaction from the very nature of a fusion process. It is thus considered that the product contains a large quantity of a free fatty acid or the starting basic metal carbonate, but the disclosure has no reference to this point. Furthermore, the process requires pre-treatments of the starting materials to prepare an aqueous emulsion of the fatty acid and an aqueous dispersion of the basic metal carbonate, and ammonia is used for the pre-treatments. A washing step is therefore required in practice to remove ammonia or an ammonium salt of the fatty acid, thus increasing the steps in number. Moreover, it is difficult to completely remove ammonia from the product. The odor of ammonia remaining in the product also give rise to a problem.
U.S. Pat. No. 4,307,027 describes a continuous fusion process comprising continuously feeding a fatty acid and a metal oxide or a metal hydroxide to a stirred-tank reactor, mixing them in a molten condition to conduct a reaction, continuously feeding the reaction mixture to a plug flow reactor to continue the reaction to continuously discharge a solid or pasty metallic soap from a discharge orifice. According to the process, though a metallic soap can be prepared in compact reactor vessels, the product is contaminated with large amounts of impurities in the very nature of a fusion process. More specifically, the product contains residual metal oxide or metal hydroxide starting material in an amount higher than the theoretical one by 1 to 1.5% by weight on an ash basis conversion.
Hence, while a number of proposals have been made on the continuous production of metallic soaps as stated above, a commercially practicable continuous process by which a metallic soap having high quality on the same level with the product obtained by a double decomposition process can be obtained using reactor vessels of reduced size has not yet been developed.
As described above, processes for preparing metallic soaps include a fusion process and a double decomposition process which have both merits and demerits, but the latter process has high merits of product quality and is recognized as superior as compared with the former process.
If the disadvantages of the double decomposition process can be overcome, there would be established an industrially favorable process for the production of metallic soaps. The disadvantages inherent to the double decomposition process are described in more detail as hereunder.
(a) The slurry concentration during the preparation process is as low as 5 to 13% by weight, and the process is batchwise. As a result, the volume of the reactor vessel is required about 10 times of the production volume, and also a vast amount of energy is needed.
(b) The aqueous slurry of a metallic soap after a double decomposition reaction is instable. Should a feed rate of the inorganic metal salt exceed a given value (varying depending on the kind of the metallic soap) during the reaction, the produced metallic soap shows water repellency and floats completely apart from an aqueous layer, making workability extremely poor.
(c) The metallic soap after the double decomposition reaction is contaminated with unreacted alkali soap and inorganic metal salt and a by-produced alkali metal salt or ammonium salt. These impurities cannot be removed completely even through the subsequent washing step. As a result, a water-soluble matter remains in the product, and causes water absorption of the product and turbidity on fusion of the product.